transom and other stuff

CB's solution is the tried and true method. It has the advantage of complete accessibility and inspection. It has the disadvantage of you being left with another wood transom that may eventually rot. Also, the v20 transom has a curve to it. 70wellcraft stated that his transom had been pieced together by 4" x 4" squares of plywood glassed together. He joked that wellcraft must have had a deal with a plywood scrap yard, but I think the real explanation is that the plywood pieces were used to create the curved transom.

I think I would remove as much of the wood as I could without disturbing the glass. How easy or difficult this will be will depend on the extent of the rot. If you can get it all out without heroic effort than I would go with the seacast solution or some other material. Would 5200 do?

On the other hand if the wood removal task became too arduous, I would cut out the back skin of the transom leaving about a 2 or 3" border. Then I would get every microscopic speck of wood out of there and off of the removed glass. Next, I would cut a 4 or 5 inch border out of marine plywood using the removed glass as a template. Then I would glass it up a la CB's suggestion. At this point you can fill the void with seacast or 5200 or whatever and you will have a new transom with a minimum of wood that will not be drilled into for motor mounts etc. You will only be drilling into wood to mount a transducer. Your new transom will probably outlive you. ;)